PNB 2274 Exam 4

Sensory Receptors

-responds to a particular modality of environmental stimuli
-transduce different forms of sensation to nerve impulses that are conducted to CNS
-send AP to create perceptions of the world created by the brain

Stimulus processing is usually...

conscious

Stimulus Processing - Special Senses

-Vision
-Hearing
-Taste
-Smell
-Equilibrium

Stimulus Processing - Somatic Senses

-Touch
-Temperature
-Pain
-Itch
-Proprioception

Processing is usually...

Subconscious

Processing - Somatic Stimuli

-Muscle Length and tension
-Proprioception

Processing - Visceral Stimuli

-Blood pressure
-Distension of gastrointestinal tract
-Blood glucose concentration
-Internal body temperature
-Osmolarity of body fluids
- Lung inflation
-pH of cerebrospinal fluid
-pH and oxygen content of blood

Functional Categories of Sensory Receptors

-Thermoreceptors
-Mechanoreceptors
-Nociceptors
-Chemoreceptors (both general and special)
-Photoreceptors (only in special sensation)

Thermoreceptors

ability to detect temperature in the environment; found in skin, liver, & hypothalamus

Mechanoreceptors

important receptor class for the somatosensory system; well known role in tactile feedback from skin and skeletal system; pain
Ex: touch receptor in the skin

Nociceptors

Alert us to potentially damaging stimuli at the skin by detecting extremes in temperature and pressure and injury related chemicals; respond when a stimulus causes tissue damage; skin, joints, tendons, blood vessels.
-Have free nerve endings
-Contains transient receptor potential (TRP) channels
*High-threshold mechanonociceptors
*Thermal nociceptors
*Chemical nociceptors
*Polymodal nociceptors
*Silent nociceptors
-Sensitized by prolonged stimulation

Chemoreceptors

sense changes in the chemical composition of the blood; info is sent from the receptor to the brain to help keep the cardiovascular and respiratory systems balanced; detecting chemicals, help maintain homeostasis

Photoreceptors

found in retina of eye; convert light into electrical signals that stimulate physiological processes; sent through optic nerve to the brain for processing

The adequate stimulus for a mechanoreceptor...
A. Photon of light
B. pH
C. Energy that is required to activate it
D. Cell stretch
E. Cold temperature

C. Energy that is required to activate it

Adequate/ normal stimulus

-The type/modality of stimulus that a receptor is most sensitive to
-Correct, appropriate, corresponding, matching

How many sensory modalities will be perceived if a sensory neuron is stimulated?

One!
-Allows brain to perceive the stimulus accurately under normal conditions

Structural Categories of Sensory Receptors

- Simple: Dendritic endings
- Complex

Simple: Dendritic endings Structure

-Free: Pain, temperature, smell
-Encapsulated: Pressure, touch

Complex Structure

-Rods and cones: sight
-Hair cells: hearing, equilibrium
-Modified epithelial cells: taste

Receptors can also be classified by "where stimuli are transduced T/F?

True

Classifications of where stimuli are transduced

-Exteroceptors
-Interoceptors
-Proprioceptors

Physiology of Receptor Potential

-Graded potential (transduction site) usually depolarizing
-Amplitude of receptor potential correlates to stimulus intensity
-If receptor membrane reaches threshold --> APs in afferent neuron (trigger zone)

Coding of Stimulus Intensity

Problem: How was stimulus intensity coded?
Solution:
1. Analog - to - Digital Conversion
2. Recruitment: Stronger stimuli "call in" additional afferent neurons

Coding of Stimulus Type

-Modality: Type of receptor stimulated
-Specifity of ascending pathway: Dorsal (posterior) column pathway more specific

Coding of Stimulus Location Punctuate Distribution

-How well can you discriminate two stimuli from one?
-How to test: Two-point discrimination
(Fingers, hand, face - especially good at two point discrimination and stimulus localization)

Coding of Stimulus Location

-Receptive field size
-Receptor density and overlapping receptor field
-Specificity of ascending pathways
-Lateral Inhibition

Receptive Field

-(for somatosenation) = that part of body which, when stimulated activates that afferent neuron

Presynaptic Inhibition

1.Action potential arrives
2.GABA release
3.Inactivation of calcium channels

1.Action potential arrives
2.Less calcium enters
3.Less neurotransmitter released
4.Reduced effect on postsynaptic membrane

Sensory Adaptation

Tonic receptors: (slowly adapting)
-Produce constant rate of firing as long as stimulus is applied (rate may slowly decrease)
Ex: pain
Phasic receptors: (rapidly adapting)
-Bursting of activity but quickly reduce firing rate if stimulus maintained.
-Sensory adaptation: Cease to pay attention to constant stimuli
Ex: pacinian corpuscle

Which of the following could be responsible for a receptor adapting to a stimulus?
A.Opening of Na+ channels
B.Closing of K+ channels
C.Opening of both Na+ and K+ channels
D.Opening of K+ channels

D.Opening of K+ channels

The functional and anatomical organization of sensory processing networks (hiearchial)

1.Pain, temperature, and coarse touch cross the midline in the spinal cord
2.Fine touch, vibration, and proprioception pathways cross the midline in the medulla
3.Sensory pathways in synapse in the thalamus
4.Sensations are perceived in the primary somatic sensory cortex.

Dorsal Column System (Tracts)

Fasciculus Gracilis + Fasciculus Cuneatus
-Ascend ipsilaterally
-Synapse and coss in medulla (nucleus gracilis, nucleus cuneatus)

Medical Lemniscus
-Ascends to thalamus + synapses

Third order neurons ascend (internal capsule) to S1

Anterolateral System

Tracts
-Spinothalamic, spinotectal, spinoreticular
-First order neurons synapse w/ second order neurons in spinal cord, and cross before ascending

Info transmitted about:
-Pain, temperature, "crude" touch

Pain Fibers

-Aδ fibers
-C fibers

Aδ fibers

-myelinaed axons
-mostly mechanical specific
-Small receptive field
-Sharp/Prickling Sensation

C fibers

-Small unmyelinated axons
-Slow conduction (0.5-2 m/s)
-Large receptive field
-Burning of aching

Gate-Control Theory

Non-noxious input suppresses pain
Application: transcutaneous electrical nerve stimulation

Pain Pathways

-Neospinothalamic tract
-Paloespinothalamic tract
-Archispinothalamic tract

Modulation of Pain Transmission

Periaqueductal Gray and Raphe nuclei (RN)
-Application: Stimulation Produced Anaglesia

Cornea

-Shields the eye from germs and dust
-Outermost lens to focus light (65-75% of focusing)

Iris

-Colored disc inside the eye
-Separates the cornea from the lens
-Creates two chambers
-Anterior chamber (between Cornea & Iris)
-Posterior chamber (between Iris & lens)

What determines eye color?

The iris determines eye color

Two pigments
-Melanin
-Lipochrome
No pigment
-pink
Some pigment
-Blue

Iris controls amount of light entering eye... T/F?

True

Where would you expect to find an inhibitory synapse such as the one illustrated in figure? (hint: where would you expect to see a synapse?)
A) Fasciculus cuneatus
B) Nucleus cuneatus
C) Between Meisner's corpuscles
D) All of the above

B) Nucleus cuneatus

What loss would result from cutting through the right side of the medial lemniscus?
A)Loss of pain sensation on the left side of the body
B)Loss of pain sensation on the right side of the body
C)Loss of fine touch sensation on the left side of the body
D)Loss of fine touch sensation on the right side of the body
E)Complete paralysis on the left side of the body

C) Loss of fine touch sensation on the left side of the body

Factors that Activate Nociceptors

* Damaged tissue releases:
-Globulin and protein kinases
-Arachidonic acid (through prostaglandin)
-Histamine
-Nerve growth factor
-Substance P
-potassium K+
-Serotonin, acetylcholine, low pH, ATP

To focus on a nearby object,
A. The lens is more rounded
B. The lens is more flat

A. The lens is more rounded

Changes in the Lens Shape

Ciliary muscle can vary its aperture

Changes in the Lens Shape - Distance Increases

-Ciliary muscle relax
-Tension on the suspensory ligament
-Lens is less convex

Changes in the Lens Shape - Distance Decreases

-Ciliary muscles contract
-Reducing tension on suspensory ligament
-Lens more convex

To focus on a nearby object, (choose all that apply)
A. The lens is more rounded
B. The lens is more flat
C. Ciliary muscles contract
D. Ciliary muscles relax
E. Tension on suspensory ligament decreases
F. Tension on suspensory ligament increases

A. The lens is more rounded
C. Ciliary muscles contract
E. Tension on suspensory ligament decreases

Retina

* Process light
-Convert visual stimulus into neuronal activity

*Neural tissue
-Communicate with the brain

Majority of the blindness due to retinal defect

Vitreous

Clear gel between the lens and the retina. Has to be transparent for light to be traverse.
Composition
-99% H2O
-Network of collagen fibers
-Large molecules of Hyaluronic acid
-Peripheral cells
-Inorganic

Rods & Cones

Rods more abundant than cones.
Cones packed in fovea.
Rods more sensitive to scatter light.
Cones more sensitive t direct light and requires more light to function.

Loss cones -> color blindness
Loss rods -> night blindness

Pigments: Rhodopsin, Cone opsin

Outer Segment of Photoreceptor
Rods: Rhodopsin
Cones: Cone opsin
-Three types; three visual pigments
-Color vision + Acuity
Composition:
-Retinal (Vitamin A)
-Opsins (Protein)
Retinal Conformations:
-Dark: 11-cis retinal (resting)
-Light: 11-trans retinal (activated: "bleaching")

Transduction: Light to AP

Light: Hyperpolarization neurotransmitters NOT Released
Dark: Depolarization neurotransmitters Released

Light is transducer via second messenger system.
1.cGMP
2.CNGs (Sodium + Calcium)
3.Phosphodiesterase
4.Transducin

When a rod cell in the eye is stimulated by light:
A. the cell membrane become depolarized.
B. the inactive form of retinal associates with bleached opsin.
C. transducer activity decreases.
D. the intracellular level of cGMP decreases.
E. more neurotransmitters are released

D.the intracellular level of cGMP decreases

Bipolar cells

*Basis for brightness and contrasts
-On bipolar cells
*detect light objects in a darker background
-Off bipolar cells
*detect dark objects in a lighter background

*Do not generate action potential (Respond to glutamate with graded potentials)

Retinal Ganglion Cells

*Their axons form the optic nerve
*Detect shape and movement
Type P (Parvi)
Type M (Magni)

Type P (Parvi)

-Color sensitive
-Small concentric receptive field
-Slowly adapting

Type M (Magni)

-Larger than P cells
-Synapses with many bipolar cells
-Color insensitive
-Fast adapting (sensitive to moving objects)

Visual Processing

1. Receptors
Rods Cones
↓ ↓
2. Bipolar Cells − the basis for brightness and color contrasts
↓ ↓
3. Retinal Ganglion Cells → Enhance contrast and the basis for movement

Horizontal Cells

The dendrites of a horizontal cell synapse with surrounding photoreceptor cells (colored green). The axon terminals synapse with one group of photoreceptors (colored red)

Amacrine Cells

-Synapse with bipolar cells and ganglion cells
-Provide lateral connections
-More than 20 different types of amacrine cells with different functions

Receptive Field is...

Determined Experimentally
Experimental Protocol:
-Subject fixes gaze on center spot
-Light stimulus (bar, spot, etc.) projected on screen
-Record from desired cells (Receptors, Retinal Ganglion Cell, LGN, primary visual cortex, etc.)
-Identify receptive field (place on screen that alters cell's firing)

Receptive Field: Retinal Ganglion Cell

Each cell responds to contrast between dark and light from a specific spot in the visual field.
Two types of RGCs:
-On-center
-Off-center

Based on what we have discussed, This recording is from an...
A.On-center cell
B.Off-center cell

B.Off-center cell

RF for Primary Visual Cortex

Visual Cortical Cell Recording
-Rectangular receptive fields
-Orientation selectivity
*Best ("adequate") stimulus is bar of light oriented at precise angle
-Most cells binocular

Primary Visual (Striate) Cortex

Visual world represented on V1 as an initial, 2-D "primal sketch"
-Outlines/boundaries delineated by short line segments
-NO color, depth, form, or motion
Information concerning form and orientation of stimulus is added.
Cortical cells' receptive field characteristics: much more "choosy" with respect to adequate stimulus.

Association Visual (Extrastriate) Cortex

Information from V1 sent to >25 extrastriate visual areas for higher processing

Sound Wave

characterized by frequency and intensity

Frequency

-Measured in hertz (cycles per second)
-Pitch is directly related to frequency
-Human hearing frequency range: 20-20k Hz

Intensity (loudness)

-Directly related to amplitude of sound waves
-Measured in decibels

The Ear

Translate sound waves which are characterized by frequency (pitch) and intensity (loudness)

Neospinothalamic tract

Responsible for immediate awareness of the exact location of the painful stimulus

Paleospinothalamic tract & Archispinothalamic tract

-Bilateral innervation
-Targets include brain stem, hypothalamus, and limbic system

Tympanic Reflex

Loud sounds initiate the tympanic reflex
-Tensor tympani and stapedius muscles contract
-Sound transmission decreases

Cochlea

Spiral cavity of the inner ear containing the organ of Corti, which produces nerve impulses in response to sound vibrations

What happens as sound waves move the fluids in the cochlea ?

The basilar membrane vibrates and the stereocilia of the hair cells are bended.

Hair Cells

Mechanical Transducers for Hearing and Vestibular System
-Graduated size
-Bend toward tallest stereocilium (kinocillium)→Depolarize
-Bend away → hyperpolarize

Hair cells are in a unique ionic environment...T/F?

True

Endolymph

-High [K+]
-Low [Na+]

Perilymph

-Low [K+]
-High [Na+]

The organ of corti is located in the ___ , which is filled with fluid that has an abnormally high ___ .
A. Cochlear duct (scala media); K+
B. Tympanic duct (scala tympani) ; K+
C. Vestibular duct (scala vestibular) ; Na+
D. Corti duct (scala corti) ; Na+

A. Cochlear duct (scala media) ; K+

When the stereo cilia of the hair cell are displaced toward the kinocilium,
A. Hair cell depolarized due to a K+ and Ca2+ influx.
B. Hair cell hyper polarized due to a K+ and Ca2+ efflux.
C. Hair cell depolarized due to a Na+ influx
D. Hair cell hyper polarized due to Na+ efflux
E. No change in membrane potential

A. Hair cell depolarized due to a K+ and Ca2+ influx

Inner Hair Cells

The primary sensory receptors

Outer Hair Cells

-Shorten when depolarized
-Increase the amplitude and clarity of sound

How are different tones coded by the cochlea?
A. By changing the amplitude of a graded potential.
B. By different regions of the cochlea.

B. By different regions of the cochlea

Frequency Coding

-Achieved through a variety of mechanisms
>Hair cells are organized tonotopically
>The pattern of the action potentials in the auditory nerve

Intensity Coding

- Frequency of the action potential
>Proportional to the loudness of the sound

-More fibers begin to respond, especially the less sensitive fibers

Major Auditory Pathways

-Dorsal and ventral cochlear nuclei
-Superior olive
-Inferior colliculi
-Medial geniculate body
-Auditory cortex

Dorsal and ventral cochlear nuclei

-Second order neurons
-Respond from one ear

Superior olive

-Respond from one ear

Inferior colliculi

-Centers for auditory reflexes

Auditory cortex

-Tonotopic
-Very plastic
-Bilateral damage results in
>distinguish frequency or intensity
>Localize sound
>Understand speech

Superior olive in Sound Localization

-Loudness difference
-Detectable time difference

Loudness difference

-Lateral superior olive
-Most important for frequencies above 3000 Hz

Detectable time difference

-Medial Superior Olive
-Most important for frequencies below 3000 Hz

Lateral Superior Olive (LSO)

encode sound location through interaural intensity differences (also called "interaural level disparity - ILD")

Medial Superior Olive (MSO)

computes location of sound by interaural time differences